1. Field of the Invention
The present invention relates to hydrofoil supported watercraft and more particularly pertains to systems for adjusting a hydrofoil's angle of attack in order to maintain the craft in a desired attitude relative to the water's surface.
2. Description of the Prior Art
The use of hydrofoils for supporting a moving watercraft has had wide consideration in the recent past. These considerations stem from the hydrodynamic advantages inherent in a hydrofoil. Specifically, the several drag components associated with conventional displacement hull designs and even planing hull designs generated substantial power requirements for propulsion which ultimately limits the watercraft's speed and efficiency. Hydrofoils, adapted to lift all or a portion of a watercraft's hull clear of the water serve to alleviate hydrodynamic drag otherwise associated with the hull while the surface drag or form drag generated instead by the hydrofoils is comparatively minimal due to the foil's relatively small surface area and clean form. That component of drag generated by a hydrofoil known as induced drag or drag-due-to-lift, can further be reduced by increasing the foil's aspect ratio.
The minimal amount of hydrodynamic drag generated by hydrofoil suspended watercraft clearly enables available power to be used more efficiently. Such enhanced efficiently can manifest itself in terms of higher speed capabilities, reduced fuel consumption rates or can be exploited by the fitment of smaller powerplants without compromising a craft's performance ability. A reduction in drag is particularly attractive for windpowered watercraft for which typically only a limited amount of useable power is available as structural as well as dynamic considerations restrict the amount of sail area that can practically be exposed to the wind while light winds reduce the amount of power available altogether. A large variety of foil systems have consequently been fitted to both sailboats as well as power boats.
The amount of lift generated by a hydrofoil is dependent upon both its velocity through water as well as its angle of attack. Higher velocities and/or larger attack angles serve to increase lift. In order to limit the amount of lift, so as to maintain an optimal degree of submersion and thereby minimize drag, a variety of systems have been devised that automatically adjust the foil's angle of attack as a function of its submersion. Examples of such systems are set forth in U.S. Pat. Nos. 3,762,353, and 4,711,195 as well as British Patent No. 591,933 wherein leading buoyant or planing members are employed to gauge the foil's submersion. These gauging members follow the water's surface contour and are directly linked to a mechanism that increases the foil's attack angle as a function of the foil's position below the reference plane defined by the gauging members' position. While the described systems are suitable for their intended purposes, the efficiency and stability of craft employing such systems breaks down as velocity increases because the gauging members attempt to doggedly follow the local wave contour. The local wave action is perceived as undulating at higher and higher frequencies as the craft's speed increases and as a result, exaggerated or unnecessary adjustments of the foil's attack angle are made in response to the gauging member's movements. A system is therefore called for that adjusts foil angle in response to the craft's general attitude relative the overall water surface rather than in response to local wave contour in order to provide a more stable suspension of the craft at high speeds.
Systems employing multiple adjustable hydrofoil surfaces have been adapted to watercraft in an attempt to control rolling forces and more specifically, have been used to control the healing of windpowered watercraft. A representative example is described in U.S. Pat. No. 4,949,695. By adjusting the windward hydrofoil's attack angle to produce negative lift while adjusting the leeward hydrofoil's attack angle to generate positive lift the crank is maintained in a more upright orientation. This not only serves to reduce the tendency to capsize, but maximizes the surface area exposed to the wind. Higher speeds can therefore safely be attained. Disadvantages associated with prior art designs are inherent in the manner in which the foils' attack angles are controlled wherein stability and effectiveness quickly diminishes as higher and higher speeds are attained. The previously disclosed systems would appear to either react too slowly or inappropriately as in the case of manually operated systems or in systems wherein the foils' attack angles are linked to the position of the boom. Conversely, systems employing linkages that are directly responsive to the foils' submersion level below the local surfaces contour suffer from the same destabilizing effects at higher speeds that were discussed above. A hydrofoil suspension system is therefore called for that more effectively controls a watercraft's rolling motion at high speeds.